14 TWELFTH ANNUAL REPORT. 



[sic?] or by high winds, hatched young, which under favorable con- 

 ditions, changed into Sida, Moina, and Daphnia-like forms. The 

 Cladocera are, then, probably the more generalized forms, from 

 which the Phyllopods, at this time and probably ever since Devo- 

 nian times, i^cif excellence a fresh-water assemblage of forms, took 

 their origin," Whatever affinity there may be between the shelled 

 Phyllopods and the Cladocera, it would seem that the evidence is 

 conclusive that the latter group is not the direct continuation 

 of the line of development inaugurated by an ostracode ancestor. 

 As shown beyond, the present centre of the group seems near Moina 

 with indications of a divergence from this rather generalized type, 

 especially of degradation and heteronomy on the side of the 

 Lynceids. 



It seems at the present time that more might be accomplished 

 for aetiology by a careful study of such groups as the present, in 

 which are a variety of closely allied forms than by the attempt to 

 join widely separated groups. When we shall have siezed upon 

 the latest eddies and mapped their dilfection, it may become possible 

 to combine the indications in such a way that lines of divergence 

 thus traced accurately through some small part of their course 

 may be produced backward to their intersection. This then is 

 our present duty — the accurate mapping of minute districts and the 

 careful noting of any moving straws, competent to indicate move- 

 ments in the vast complex of vitalized nature. We conceive the 

 cladocera to have had a comparatively recent origin, and to express 

 the culmination and retrograde development of a plan of structure 

 first differentiated after the appearance of clear bodies of fresh 

 water. All the species save a very few are confined to inland 

 waters. Accepting the above mentioned theory, the Sididas will 

 occupy the first place as departing least from the type from which 

 the whole group sprang, while it is connected by the genus 

 Daphnella with the Daphnidas. The Daphnidas, beginning with 

 Moina, find their ultimate development in some monstrous forms 

 of the genus Daphnia, but pass into the Lyncodaphnidae by way 

 of Macrothrix. The links uniting all these minor groups are very 

 obvious. 



Our own ideas of the relationships among the Calytomerous 

 Cladocera are expressed in the accompanying table. This table is 

 to be considered a projection of a portion of a genealogical tree, 

 seen from below, in which the genus Moina forms the arbitrarily 

 chosen fixed point. The heavy dotted line is imagined as directed 

 downward vertically. That branch rising toward the top of the 



